Not Applicable
Not Applicable
The invention relates to a driving system for industrial trucks.
The primary drive for industrial trucks either is an internal combustion engine or an electric motor with the motor being fed by a battery. Either driving mode has its advantages and disadvantages. Industrial trucks using combustion engines cannot be used permanently in closed rooms because of the emission of harmful substances. Therefore, electric motor driven industrial trucks are provided for such applications. For such drives, however, it is necessary to carry along a large and heavy battery which needs to be charged from time to time. The explanations which follow and the invention exclusively refer to industrial trucks which have a combustion engine.
Industrial trucks not only require to be driven, but also require that working energy be provided for most various functions, especially for lifting and lowering the load-carrying means, for inclining, pushing, and rotating the lift frame as well as for driving specific attachments.
Like in other power-driven vehicles, a transmission is required in employing combustion engines for industrial trucks in order to obtain the desired torque and the desired number of revolutions. Using a change-speed transmission including a clutch, however, is very unfavourable for industrial trucks. Therefore, it hardly has an application in practice.
Another alternative is to use a hydrodynamic transducer in combination with single-step or multi-step reduction gears. This solution proves to be disadvantageous at slow travelling speeds because efficiency is very unfavourable in this operating state. Moreover, a clutch function is required to independently operate the lifting function.
Another alternative is to use a hydrostatic transmission. This allows relative high efficiency at a slow travel, but involves low efficiency at high travelling speeds. More drawbacks are the relatively high construction expenditure which is due to the high hydraulic pressures up to 400 bar and the system""s susceptibility to defects, especially with regard to leakage.
Finally, it is also known to perform the transmission of forces by means of an electric transmission. In this one, all the energy generated by the combustion engine is converted into electric energy by means of a generator. Then, this one is reconverted into mechanical energy by means of an electric motor. Drive controls for electric motors are capable of adjusting any gear ratios between the generator and the electric motor. Hence, a system of this type has the advantage of being controllable particularly well. A drawback, however, are the relatively low efficiency due to the high losses in the controls and electric machines.
The existing driving systems for industrial trucks involve considerable energy losses in certain operating situations. Furthermore, the known systems are incapable of returning energy in braking. Furthermore, emissionless operation is impossible. Another disadvantage of the known driving systems is that the components of the systems require to be designed for the peak load. The mean load acting on the components, however, clearly is below such values. Thus, a large construction expenditure is incurred that is not needed for the mean.
DE 198 03 160 has made known a two-branch driving system for power-driven vehicles. It is intended to meet the requirements to an automobile in a stop-and-go operation in conurbations. The known hybrid drive for power-driven vehicles using a combustion engine provides a first electric machine disposed on a driving shaft of the hybrid drive and a second electric machine the two of which are always operable as a generator and an electric motor and are coupled to each other and to the combustion engine via a two-branch gear transmission. The second electric machine is positioned on a shaft with which a rotary brake is associated which is controllable in dependence on the driving condition of the power-driven vehicle and by which the respective shaft can be located. A stop or arrest of this electric machine with the matching driving shaft is performed especially if the speed thereof falls below a certain limit, i.e. if the second electric machine has a low partial efficiency. This helps avoid overheating the electric machine. The known driving system, however, is unsuitable for industrial trucks since it does not make possible a backward travel at a maximum speed.
DE 197 47 459 has made known a driving system for tractors and agricultural machinery in which the combustion engine is connected, via a hydrostatic transmission, to a summing transmission the further input of which is driven via a second drive branch which is coupled to the driving shaft of the combustion engine via a branch gear transmission. The driving system is intended to utilize the advantages of a hydrostatic drive and a mechanical drive in combination in order to improve the usability of the vehicle drive. This driving system requires two clutches and does not make possible any backward travelling speed at a maximum speed.
It is the object of the invention to provide a driving system for an industrial truck that realizes optimum modes of operation for all driving conditions at an acceptable construction expenditure and a high efficiency and that also enables to be driven within buildings without any exhaust gases.
In the inventive driving system, a combustion engine is used as a primary motor and an electric motor is used as a secondary motor. For example, the output shaft of the electric motor directly acts on a gear assembly the input of which, in addition, is coupled into the shaft of the combustion engine and the output shaft of an adjustable transmission. Connected after the gear assembly, which also can be designated as a summing transmission, is a change-over transmission in order that the forward and backward travels of the industrial truck may be performed under equal conditions. A sensor system measures the speed of the combustion engine, the electric motor, and the output shaft of the change-over transmission and generates respective speed signals for a governing device which controls the speeds of the motors and the gear ratio of the adjustable transmission according to a desired-value signal of a desired-value transmitter actuated by the operator of the industrial truck.
As adjustable mechanical transmissions, various infinitely variable transmissions are imaginable such as a V-belt driven variable transmission, a steel thrust belt driven variable transmission, a steel thrust chain variable transmission, a friction gear variable transmission, an adjustable hydrostatic transmission or a hydrodynamic transducer (a Foettinger coupling).
Various variants are imaginable to realize the gear assembly. In an aspect of the invention, a first planetary gear system such as a planetary mechanism is coupled to the shaft of the combustion engine and a second planetary gear system such as a planetary mechanism is coupled to the driven shaft of the adjustable transmission. The output shaft of the one planetary gear system defines the input shaft of the second planetary gear system and the electric motor is coupled to the input shaft of one of the two planetary gear systems with the torques of the three shafts of the planetary gear systems being at a fixed ratio to each other. The speeds of the shafts adjust themselves in dependence on the operating state.
Alternatively, a provision is made that a planetary gear system be coupled to the shaft of the combustion engine and the output shaft of the adjustable transmission and the output shaft of the planetary gear system and the shaft of the electric motor define the input shafts of a further planetary gear system. Finally, in a further aspect of the invention, a first planetary gear system may be coupled to the shaft of the combustion engine and the shaft of the electric motor whilst its driven shaft defines the input shaft of the adjustable transmission. The driving shafts of the adjustable transmission and the first planetary gear system define the input shafts of a second planetary gear system.
In all aspects of the invention, the power of the combustion engine is branched by the adjustable transmission onto a primary shaft and a secondary shaft. As the primary shaft, the continuous shaft may be referred to whereas the secondary shaft represents the driven-side shaft of the adjustable transmission. Alternatively, the branching of power to the primary and secondary shafts may be performed by a separate two-branch gear transmission. Then, the adjustable transmission is arranged in the secondary shaft. The powers of the primary and secondary shafts will be led together again in the planetary gear system and the gear assembly. Such a driving system has several advantages:
a) The maximum power to be transmitted by the adjustable transmission is reduced. Transmissions having a variable gear ratio generally are of an efficiency which is lower than the one of those having a constant gear ratio. Therefore, the power loss decreases and the overall efficiency of the drive increases
b) Because of the lower demands to power, adjustable drives may be used the capacity of which would not be sufficient otherwise, e.g. V-belt variable drives. In addition, the adjustable drive may be dimensioned smaller.
c) The gear ratio between the combustion engine and the output shaft of the driving system may be made to be zero. This enables the vehicle to be kept at stoppage while the combustion engine is running with the flux of power not being interrupted by an engaging and disengaging clutch. In addition, the spread of the gear ratio takes on any magnitude desired in this manner. (The spread referred to here is known to be the relationship between the largest and the smallest gear ratio of an adjustable transmission).
However, a driving system possesses the two first advantages only if a change-over transmission is provided for the reversal of the sense of rotation when a change is made between the forward and backward travels. It is true that the reversal of the sense of rotation could even be realized without any change-over transmission, but solely by varying the gear ratio of the adjustable transmission. Without any change-over transmission, the maximum power to be transmitted by the adjustable transmission at a backward travel at a maximum speed would be larger than the output power of the driving system""s driving shaft.
Because the two-strand driving system is coupled to an electric motor secondary drive (an electric machine which may be operated both as a motor and a generator) the advantages below will emerge:
The starter and the light generator may be omitted.
A purely electrical operation is possible, e.g. driving in closed rooms.
Energy recovery may be effected in braking. This reduces the consumption of energy and the requirements to the thermal load-carrying capacity of the operating brakes are lower.
Exerting a rapid, highly dynamic influence on the driving torque and the number of drive revolutions is possible without any problems for a control of the travelling speed. The requirements to the adjustable transmission with regard to the dynamics of variation and the accuracy of the gear ratio are substantially lower.
The combustion engine may be dimensioned smaller because peak power requirements can be met by the additional power of the secondary motor.